This invention pertains to a method of preparing hexaalkylguanidinium salts.
Hexaalkylguanidinium salts are useful as phase transfer catalysts for reactions between highly polar reagents, such as alkali metal salts of hydroxyaromatic compounds or thio analogs thereof, and substantially non-polar reagents such as activated halo- or nitro-substituted aromatic compounds. Typical nucleophilic aromatic substitution reactions of this type result in replacement of the halo or nitro group with an aryloxy or arylthio group. Such nucleophilic aromatic substitution reactions are particularly useful commercially for the preparation of aromatic ether bisimides, such as 2,2-bis[4-(dicarboxyphenoxy)phenyl]propane bisimides and 4,4'-bis(dicarboxyphenoxy)biphenyl bisimides. These bisimides may react directly with diamines to produce polyetherimides, as disclosed, for example, in U.S. Pat. No. 4,578,470. Alternatively, these bisimides may be converted to dianhydrides, which in turn also react directly with diamines to produce polyetherimides. The analogous monoimides are similarly useful as endcapping or chain-stopping agents for polyimides.
Only a few methods of preparing hexa-substituted guanidinium salts are known. One such method, as disclosed by A. V. Santoro et al. in the Journal of Organic Chemistry, 44, 1979, 117-120, teaches the reaction of tetramethylguanidine with two equivalents of benzyl halide to yield 2,2'-dibenzyl-1,1,3,3-tetramethylguanidinium halide. Disadvantageously, this reaction does not go to completion, and a mixture of tetraalkyl-, pentaalkyl- and hexaalkyl-guanidines is obtained. Fractional recrystallization is therefore required to isolate the hexaalkylguanidinium salt. It is noted that this reference is silent with regard to the use of base and phase transfer catalysts.
A second method of preparing hexa-substituted guanidinium salts involves reacting pentaalkylguanidine with one equivalent of alkylating agent, as disclosed by W. Kantlehner et al. in Liebigs Ann. Chem., 1984, 108-126. This preparation is outlined in Scheme I: ##STR1## wherein R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, and R.sup.6 represent alkyl groups typically having up to about 20 carbon atoms, and wherein X.sup.- is a halide, such as chloride, bromide or iodide.
Disadvantageously, the process of Kantlehner et al. requires a prior synthesis of the pentaalkylguanidine. This synthesis is itself a two-step procedure, as disclosed by P. Pruszynski in the Canadian Journal of Chemistry, 65, 1987, 626-629. Pruszynski teaches that 1,1,3,3-tetramethylurea reacts with phosphorus oxychloride to give a phosphorus oxychloroformamidinium salt. The latter reacts with aniline, or a derivative thereof, to give the corresponding 2-phenyl-1,1,3,3-tetramethylguanidinium salt, which is thereafter treated with base to recover the penta-substituted guanidine, 2-phenyl-1,1,3,3-tetramethylguanidine, or a derivative thereof. Pruszynski's approach is outlined in Scheme II: ##STR2##
An analogous synthesis of penta-substituted guanidines, disclosed by D. H. R. Barton et al. in the Journal of the Chemical Society, Perkins Transactions I, 1982, 2085-2090, involves the reaction of tetra-alkylureas or tetra-alkylthioureas with phosgene. The initial product is a chloroformamidinium salt analogous to the one prepared from phosphorus oxychloride. The phosgene-derived salt reacts similarly with primary amine to give the penta-substituted guanidinium salt, which is likewise neutralized to the corresponding penta-substituted guanidine.
Other methods are reported by Kantlehner et al., op. cit., for the preparation of hexa-substituted guanidinium salts. For example, it is disclosed that N,N,N',N'-(tetramethyl)thiourea reacts with N,N'-dimethylcarbamoyl chloride to give hexamethylguanidinium chloride. Also disclosed is the reaction of N,N,N',N'-(tetramethyl)thiourea with dimethyl sulfate to yield a carbenium methylsulfate, which in the presence of dimethylamine affords hexamethylguanidinium methylsulfate. Neither of these reactions is practical because expensive starting materials are employed.
It would be desirable to have a practical method of preparing hexaalkylguanidinium salts. More desirable would be a method which achieves a high yield of the guanidinium salt, and does so without extraordinary purification procedures, such as fractional crystallization. Such a method would facilitate the commercial preparation of numerous hexa-substituted guanidinium salts, which could then be employed as phase transfer catalysts in a variety of nucleophilic aromatic substitution reactions.